This invention relates to an oil control ring attached in a ring groove of a piston in an internal combustion engine, in particular to a two piece oil control ring that has a coiled expander and a ring body with nitrided surface layers formed thereon.
Various types of coil expander loaded oil control rings have generally been utilized in most of diesel engines. A typical example of two piece oil control rings comprises, as shown in FIG. 6, a ring body 1 with an annular groove 4 on an inside surface 3 of ring body 1; and a coiled expander 2 attached in annular groove 4 of ring body 1. Ring body 1 comprises an upper rail 5, a lower rail 6 and a web 7 that connects upper and lower rails 5, 6 to form a generally M-shaped section. A plurality of openings 8 are formed in web 7 to allow lubricating oil to pass through the openings 8.
Not shown but, wear resistant surface layers are formed on outer peripheral surfaces of upper and lower rails 5, 6, and on inner peripheral surfaces of ring body 1 with annular groove 4. Coiled expander 2 can produce elastic force to resiliently urge each outer peripheral surface of upper and lower rails 5, 6 toward an inner wall of a cylinder not shown. Cross sectional shape of ring body 1 is determined in considering followability in operation of deformable ring body 1 to inner circumferential surface of cylinder, tangential force of oil control ring, stress of ring body 1 produced when attached in a groove formed in piston, running stress and workability of wire rods into rings so that the oil control ring can perform such essential function as to apply an appropriate amount of lubricating oil on the inner surface of cylinder. In response to recent requirement of higher power and lower fuel consumption for diesel engines, ring bodies are formed of steel in lieu of cast iron, and wear resistant surface layers are formed by nitriding instead of hard chromium plating to improve wear and scuff-resistance of ring body.
Japanese Patent Disclosure No. 6-336951 shows an oil control ring that has a ring body made of 17% chromium stainless steel with ion-nitrided layers formed on outer and inner circumferential surfaces of ring body. In making such an oil control ring with ion-nitrided layer, a drawing process is used to produce a deformed wire rod of a given usual cross section, and then a plurality of openings are formed in a web of the wire rod by punching. Then, the wire rod is formed into a coil and cut into ring bodies, lapped with outer circumferential surfaces of the ring bodies, ground with upper and bottom surfaces of the ring bodies, and nitrided with whole circumferential surface or necessary areas of the ring bodies. After nitriding, the ring bodies are ground in a similar way to finish oil rings of final shape.
Some attempts have recently been made to fabricate a two piece oil control ring made of stainless steel of low chromium content less than 17 weight % to reduce manufacturing cost. This oil control ring is, however, disadvantageous in that roundness or circularity of ring body is remarkably deteriorated with warpage upon nitriding expansion during and after a nitriding process because the process involves unbalanced stresses produced in inner and outer nitrided layers of ring body formed into M- or I-shape made of stainless steel having low chromium content. When nitrided layers are formed on whole circumferential surfaces of M- or I-shaped ring body 1, the nitrided layers are expanded to generate compression stresses in the nitrided layers, and thereby to cause tensile stresses in the base material and incidental distortion in the inner and outer circumferential sides of ring body 1. If distortion produced in the outer circumferential side is greater than that produced in the inner circumferential side, the outer circumferential side naturally expands with longer extended length than that of the inner circumferential side, deforming end portions of ring body 1 toward the inner circumferential side. When the lapping process is applied to circumferential surfaces of such deformed ring, the end portions of ring body cannot uniformly be lapped due to distortion by nitriding. When an oil control ring of degraded circularity is attached to a piston, it provides a clearance or gap between an inner surface of a cylinder and outer circumferential surface of the oil ring, thereby resulting in detrimentally increased oil consumption. Otherwise, when the whole outer circumferential surface is lapped to a required periphery circularity of ring body, the nitrided layer is disadvantageously removed by lapping, and therefore, after nitriding, lapping of the ring body is extremely restricted or should be performed very carefully. On another aspect, it also has been found that reduced chromium content deteriorates creep resistance of ring body.
FIG. 5 is a graph showing increase of lapping amount and elapse of lapping time in a proportional relationship. The term xe2x80x9clappingxe2x80x9d referred herein means any process for polishing, grinding, scouring, scrubbing or abrading a ring body with lapping agent to adjust or compensate dimensional error or to improve surface finish during manufacture. For example, a nitrided layer of 70 micrometers thickness is thoroughly removed through a lapping process of 105 seconds. Accordingly, lapping time should be within 105 seconds to surely leave nitrided layers on whole outer circumferential surfaces of ring body 1 so as to bring lapped profile of nitrided layers in ring body 1 into contact to inner surface of cylinder. A prior art ring body of 17% Cr stainless steel produces similar unbalanced stresses in inner and outer nitrided layers, but indicates very little amount of strain due to nitridation because 17% Cr stainless steel has greater mechanical strength, and whole circumferential surfaces of the ring body can be lapped by usual process without presenting any substantial problem. However, cost problem is still involved in 17% Cr stainless steel.
An object of the present invention is to provide a two piece oil control ring that has a ring body capable of keeping such peripheral circularity that nitrided layer formed on the ring body survives lapping. Another object of the present invention is to provide a two piece oil control ring with a ring body that can control or restrict amount of deformation produced when nitrided layer is formed on a surface of the ring body. Still another object of the instant invention is to provide a two piece oil control ring with a ring body made of stainless steel having low chromium content.
A two piece oil control ring according to the present invention, comprises a ring body (1) formed into a generally M- or I-shaped section with an annular groove (4) on an inside surface (3) of the ring body (1); and a coiled expander (2) attached in the annular groove (4) of the ring body (1) for resiliently urging the ring body (1) toward an inner wall of a cylinder. The ring body (1) comprises upper and lower rails (5, 6) and a web (7) that connects the upper and lower rails (5, 6), the web (7) being provided with a plurality of openings (8) for allowing lubricating oil to pass therethrough. The ring body (1) is made of stainless steel that contains chromium of 3.0 to 13.0 weight % with the outer and inner circumferential surfaces (11, 12) covered with nitrided layers (14, 15). A circular ratio (xcfx84) of an outer surface (edge) length (Yo) to an inner surface (edge) length (Yi) in the cross section of the ring body (1) is in a range between 1.08 and 1.32 to keep precise peripheral circularity of the ring body (1) with less strain or deformation by nitridation and thereby to allow the nitrided layers (14, 15) to survive lapping of the ring body (1).